Effect of geometry of real-size transparent nozzles on cavitation and on the atomizing jet in the near field

Abstract

Cavitation inception is very sensitive to the nozzle geometry. Two diesel-like transparent nozzles with sharp and round inlet edge were investigated under moderate injection pressure. By means of backlight imaging technique, images were recorded in two regions near the orifice: within ten orifice diameter downstream the nozzle and inside the nozzle, for different times after start of the injection. A cross-correlation technique was applied to images downstream the orifice to estimate the average velocity of the spray. The development of the spray was studied at the transient and full-opened stage, related to the needle lift motion. To analyse the flow inside the nozzle, the relative size of the vapour pockets was measured on the image and the relative frequency of the occurrence of cavitation was determined over image series. This approach permits to examine the time variation of the cavitation occurrence from the inlet to the outlet of the orifice. The results highlight the sensitivity of the cavitating flow to the shape of the inlet edge. Moreover, the trends of the variation over time of the velocity and of the cavitation frequency are very similar to each other but different for each geometry, indicating that the characteristics of the diesel spray in the near field is closely affected by the way cavitation is developing in the nozzle. Keywords diesel injection, index matching, backlight, liquid interface tracking, frequency of cavitation IntroductionFuel consumption and engine emission are important issues concerning transportation industries. Engine optimization still needs to be improved, not only to balance the global reduction of fossil fuel reserves, but also to reduce their environmental impact. There is a pressing need to address clean combustion objectives, by developing a better understanding and control of every process steps of internal combustion engines. Atomization of fuel is a key element as it impacts the whole combustion process by affecting the initial conditions of this process. In modern diesel engines, atomization efficiency was improved by considering high injection pressure in combination with small orifice diameter of only about a hundred micrometres or less. Large pressure variations of the fuel flow occurring inside injector nozzles promote cavitation occurrence. Indeed, cavitation occurs when the relative pressure drops locally below the vapour pressure of the fuel or when local stress is very high, resulting in the formation of vapour pockets or cavities in the nozzle orifice [1]. Nurick [2] proposed the first theoretical model to predict nozzle flow under cavitation, based on two-dimensional transparent single-orifice nozzles. Since then, similar nozzle designs were used to analyze the development of the cavitating flow and improve the model, by means of experiments [3,4,5,6,7] and simulations [8,9]. The flows in the nozzle were compared by changing either the L/D ratio or the inlet geometry of the orifice. Cavitation inception was found to be very sensitive to the nozzl...

Experimental and Analytical Study of under Water Pressure Wave Induced by the Implosion of a Bubble Generated by Focused Laser

“…The spray formation depends greatly on the flow characteristics at the nozzle orifice outlet, such as the turbulence energy and velocity distribution. The presence of cavitation within an orifice can also enhance the spray characteristics [ 6 , 7 , 8 , 9 , 10 ]. Some medical therapies take advantage of the mechanical impact of inertial cavitation, such as thromboltic therapy [ 11 , 12 ] and shock wave lithotripsy (SWL) [ 13 , 14 , 15 , 16 , 17 , 18 ].…”